KeepIf Command

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KeepIf( <Condition>, <List> )
Creates a new list that only contains those elements of the initial list that fulfil the condition.
Example: KeepIf(x<3, {1, 2, 3, 4, 1, 5, 6}) returns the new list {1, 2, 1}.
Note: For list of numbers arbitrary condition may be used. For list of other objects one can use only conditions of the form x==constant or x!=constant.
KeepIf( <Condition>, <Variable>, <List> )
This syntax allows a more flexible condition.
Example: For Points P, Q, R KeepIf(x(A) < 3, A, {P, Q, R}) will filter the points whose x-coordinate is less than 3 out of the list. The variable A is replaced in turn with P then Q then R for the check.
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